Hip fractures are a significant cause of disability and mortality in older persons. The most common tool for diagnosis of osteoporosis and prediction of fracture risk is bone mineral density (BMD) measured by dual- energy X-ray absorptiometry. Yet, up to half of individuals suffering a hip fracture do not have osteoporosis by BMD testing; thus alternative approaches to fracture risk assessment are needed. This underscores the importance of looking for additional contributors to fracture risk, such as reduced soft tissue thickness at the trochanter, which attenuates the fall forces that may lead to fracture. According to biomechanical principles, when the ratio of applied force to bone strength, (termed the 'Factor-of-Risk'), exceeds one, a fracture will occur. Two previous studies have examined the Factor-of-Risk along with trochanteric soft tissue thickness but had only small numbers of hip fractures, and conflicting results. To better understand the factors associated with hip fracture, we propose to use a biomechanical approach to assess hip fracture risk that includes both estimated forces applied to the hip during a sideways fall as well as the strength of the proximal femur. The specific goal is to investigate the contribution of trochanteric soft tissue thickness t hip fracture risk, and to include this in the biomechanical Factor-of-Risk model. We hypothesize these factors predict hip fracture, and further that the factor-of-risk prediction of hip fracture isk will prove better than BMD assessment alone and better than the World Health Organization FRAX tool. We will identify prospectively ascertained hip fracture cases from three cohorts (Framingham Study, MrOS, Rancho Bernardo Study) and from each cohort also select up to four sex- and age-matched (within 3-years) non-fracture cohort members as controls, using the case-cohort study design to increase efficiency. Soft tissue thickness in 2,435 individuals will be assess on whole body DXA scans obtained at the earliest time in each cohort before any subsequent hip fracture occurred. Relative risks will be assessed as odds ratios using conditional logistic regression. This study has potential to significantly alter the way hip fractue risk is currently being assessed, and thus how patients are selected for drug therapy.